Methods are well known in the art for using a tubular reactor to form low density ethylene-based polymers from ethylene, and optionally, one or more comonomers, such as low density polyethylene (LDPE). The overall process is a free-radical polymerization in a tubular reactor containing a process fluid, where the process fluid is partially comprised or ethylene and the ethylene is converted to an ethylene-based polymer in a highly exothermic reaction. The reaction occurs under high operating pressure (about 1000 bar to 4000 bar) in turbulent process fluid flow conditions at maximum temperatures of about 160° C. to about 360° C. The reaction initiation temperature, or the temperature in which the monomer (and optional comonomer) to polymer conversion is initiated (or in the case where there are multiple reaction points along the reaction tube, reinitiated), is from about 120° C. to about 240° C. Typical single-pass conversion values for a tubular reactor range from about 20 to about 40 percent.
The reaction is initiated (and reinitiated) by injecting an initiator into at least one reaction zone within the reactor tube. The initiator is mixed with the process fluid and, in the presence of heat (usually latent heat—the process fluid is typically already at an adequate reaction temperature), the initiator forms free-radical decomposition products. The decomposition products start a free-radical polymerization reaction with the ethylene (and optional comonomers) to form the product ethylene-based polymer.
The reaction generates significant heat in the reaction zones. Without proper cooling, the adiabatic temperature rise in the process fluid (which now contains ethylene-based polymer, i.e., product, that absorbs and retains heat) eventually results in unfavorable and possibly uncontrollable reactions. Such undesirable reactions may include ethylene decomposition (forming products such as carbon, methane, acetylene, and ethane), formation of high molecular weight polymer chains, and termination by combination and crosslinking, which may lead to a broadening of molecular weight distribution. The results of such undesirable reactions range from a variation in product quality and consistency issues, to reaction system shutdown, venting, and cleanup.
To reduce the level of undesirable reaction products, the tube reactor is generally jacketed such that water may be circulated to provide cooling to the reaction zones. Nevertheless, due to the demanding temperature and pressure conditions in the reactor, the cooling water can corrode the outer surface of the reactor tube, and may thereby reduce the lifetime of the reactor. Thus, it would be useful to provide a tubular reactor and process for making LDPE in a tubular reactor, in which the corrosion of the outer surface is reduced, thereby prolonging the life of the reactor.